C OUPLING THE D EEP C ARBON AND S ULFUR C YCLES – I MPLICATIONS FOR THE O RIGIN AND S TORAGE OF D EEP E ARTH V OLATILES Rajdeep Dasgupta CIDER Community Workshop, CAMay 08, 2016

Volcanic degassing hazards long-term climate Bio-essential elements Origin of life Mantle melting Chemical differentiation Properties of asthenosphere + lithosphere Physical properties (wave speed; e-conductivity; viscosity; density; attenuation) Redox state Proto-atmosphere chemistry C, H, N, S Multi-valent elements Volatiles Light elements

What is a deep volatile cycle? What are the reservoirs? How deep? How much? Who are the carriers? – storage How fast does it cycle/move? – fluxes and processes

The effect of carbon (as 4+ ) on melting Beneath oceans Dasgupta (2013) Dasgupta et al. (2013) Beneath continents How much and what form?

Storage of C in the mantle

C storage = f (P, T, fO 2, bulk composition) Negligible storage in silicate minerals

Shirey et al. (2013)

Redox State, metal precipitation, and diamond in the deep mantle? Woodland and Koch (2003); Frost & McCammon (2008); Rohrbach & Schmidt (2011); Stagno et al. (2013); Dasgupta & Hirschmann (2006) Ni-rich, Fe-Ni alloy Reduced C (graphite/diamond) Both as accessory phases in the deep upper mantle? Fe 2+ ⇔ Fe 3+ Fe 2+ ⇔ Fe 3+ +Fe 0

Phase assemblages of alloy-diamond subsystem in the sub-ridge mantle at ~250 km Diamond would be stable only in C-rich mantle (subduction modified?) Can the presence of sulfide in the mantle help C precipitation? Fe-Ni-C system (data from Rohrbach et al. 2014)

Starting mix 0.1 wt.% Fe-Ni alloy in the mantle (250 km depths) Ni/(Fe+Ni)=0.61 in alloy metal 30 ppm C in the mantle 100, 150, and 200 ppm S as sulfur or sulfide(Fe 50 Ni 8 Cu 2 S 40 ) High P-T experiments Walker-type multianvil 6-8 GPa, °C MgO capsule Tsuno and Dasgupta ( EPSL)

Textures Low T: solid + liquid + metastable graphite High T: liquid + metastable graphite Tsuno and Dasgupta ( EPSL)

The addition of S lowers the T of solid alloy-out boundary S expands the stability field of liquid alloy T solidus <800 °C The effect of S on solid-out T Tsuno and Dasgupta ( EPSL)

C solubility in liquid alloy (the effect of S) S causes a decrease of C solubility in the Ni-rich liquid R14 = Rohrbach et al. (2014) Tsuno and Dasgupta ( EPSL)

Diamond Stability in Deep Sub-ridge Mantle (8 GPa, ~ 1400 °C) Diamond, coexisting with Ni- and S-rich liquid alloy can be stable in the C- depleted oceanic mantle 250 km 200 ppm S in the bulk mantle ; > ~5 ppm C for diamond formation C in the mantle (wt. ppm) 0 ppm S ;> ~55 ppm C 100 ppm S ;> ~20 ppm C Rohrbach et al. (2014) Tsuno and Dasgupta ( EPSL)

Storage of C in deep (sub-ridge) mantle In C-poor and S-poor depleted mantle most of the bulk carbon would be stored in the S-bearing alloy liquid but diamond may not be completely consumed Alloy liquid Diamond Tsuno and Dasgupta ( EPSL) Sulfide-rich alloy liquid

Origin of C and S in BSE

Origin of C-O-N-H-S Volatiles – The Initial Condition ?

Key Questions What type of impactors likely brought C, S, and other volatiles? -Differentiated vs undifferentiated? -Reduced versus oxidized? Timing of delivery? – predating core formation OR post-dating core formation?

D C (metal/silicate) decreases with increasing T, melt depolymerization, and fO 2 and increases with increasing P Dasgupta et al. (2013 – GCA); Chi et al. (2014 – GCA) D C (metal/silicate) – Fe-Ni-C melt/Basalt NBO/T ~ 1.5 (more MgO-rich)NBO/T ~

D C (metal/silicate) Effect of fO 2 Dasgupta et al. (2013 – GCA); Chi et al. (2014 – GCA)

Equilibrium Core-Mantle Fractionation of C

Wish list Deep cycles of volatiles and their interplay -In the present-day Earth -Through deep time